Flask



FIG. 1 is a bottom plan view of a flask showing my new design;

FIG. 2 is a side elevational view thereof, all side elevational views thereof being identical;

FIG. 3 is a top and side perspective view thereof; and,

FIG. 4 is a top plan view thereof.

The broken line showing of the central region of the bottom surface is for environmental purposes only and forms no part of the claimed design. 

The ornamental design for a flask, as shown and described. 